The goal of this research is to reveal the molecular composition and functions of the 26S proteasome in a multicellular eukaryote, using Arabidopsis thaliana as the model. Organisms rely on protein degradation for a variety of essential functions, including the maintenance of free amino acid pools, the removal of damaged proteins, and the control of regulatory protein abundance. The major machine for this breakdown is the 26S proteasome, an ATP-dependent 2-MDa proteolytic complex. The specific aim of this project is to define the composition of the native 26S proteasome from Arabidopsis, reveal the functions of various subunits, and determine whether the plant preferentially assembles specific subtypes of the complex through selective pairing of various isoforms. A more complete characterization of the 26S proteasome will lead to a deeper understanding of how proteins are selectively degraded which may potentially identify new ways to control protein breakdown in vivo. Given the importance of Ub and the 26S proteasome to almost all facets of eukaryotic physiology and development, this work should have practical benefits to both medicine and agriculture, including various disease caused by defects in the Ubiquitin/26S proteasome system. [unreadable] [unreadable]